1. Field of the Invention
The present invention is directed to a rotating torque measurement, and more specifically the use of a torque measurement to determine the consistency of a pulp slurry, the viscosity of a fuel oil, or in the mining industry, the concentration of a mining slurry in a grinding circuit.
2. Description of the Related Art
In the pulp and paper industry, the preparation and control of a pulp stream depends directly on the consistency of the moving pulp slurry. For example, the addition of bleaching additives, of retention aides, of various filler, starches and additives is all based on the consistency of pulp slurry. To date, the most accurate measurement of consistency is still based on a mechanical measurement of the shear forces exerted by the moving pulp stream on a sensing element.
Large boats and freighters are typically powered by large, oil-fired industrial boilers which use low grade, bunker fuel oils as the primary fuel. Bunker fuel oils are the residual grades of the petroleum distillation process and can have a very high viscosity. These types of fuel oils can only be injected into a boiler if they are pre-heated to sufficiently reduce their viscosity. The efficiency of the burning process directly depends on how well the bunker fuel oil can be made into a mist and how uniformly it can be injected into the boiler--both of these factors depend on the viscosity of the bunker fuel oil.
It has been found that both pulp consistency and fuel-oil viscosity can be determined by measuring the torque on the shaft of a rotating impeller. When consistency (or viscosity) increases, the shear forces on the rotating impeller increases and it takes a higher driving force (torque) to rotate the impeller. Commercial devices are available to affect such a measurement, such as a consistency transmitter type MEK-41 manufactured by BTG, a Swedish company, and a rotating viscosity meter manufactured by Brookfield, a USA company. The former device uses a sensing shaft concentrically mounted within a drive shaft. A shear force measuring element (impeller) is mounted on the exposed end of a "sensor shaft" which is positioned directly into the moving pulp stream or fuel-oil. The sensor shaft rotates with the same rotational velocity as the outer shaft and is loosely coupled thereto. The outer shaft provides the main rotational driving force to the impeller and shields the sensor shaft from the frictional forces between the drive shaft and the outer gasket material, which prevents the pulp slurry from entering the housing of the sensor unit.
In a typical implementation, the relative rotational motion between the inner sensor shaft and the outer drive shaft is sensed and a counter-torque is applied to the sensor shaft so that it will rotate at exactly the same rotational velocity as the drive shaft. This counter-torque is equal and opposite to the torque on the sensor shaft produced by the shear forces of the pulp slurry on the rotating impeller. In the prior art, this counter-torque is not based on an absolute measurement of torque, but rather on secondarily deduced factors as measured by an electronic transducer or a rotating pneumatic transducer operating on the flapper-nozzle principle.
While this technology has long been used and accepted by industry as the best and most accurate method of measuring consistency, it has two fatal draw-backs--lack of durability and reliability. A pulp slurry is very abrasive and corrosive. This means the gasket materials used to isolate the rotating shafts have a finite lifetime, measured in months, and when one of the gaskets fails, the entire device can fail catastrophically. Pulp slurry will then penetrate to the interior of the sensing housing and damage the sensitive elements. When a critical pulp consistency transmitter fails, the computerized process control loop goes out of control and the production line has to be shut down. For a large paper machine, the cost of such a forced shutdown is measured in the thousands of dollars per work shift. Therefore, it is important to provide a means to maintain and service a pulp consistency transmitter without having to shut down production.
Even though this invention makes specific reference to a pulp slurry and fuel oil, it should be understood that it also applies to a variety of other materials, such as natural and synthetic fibers like cotton, wool and kevlar fibers, as well as many other kinds of fluids, such as molasses in the crystallization process leading to the production of refined sugar.